Field of the Invention
Embodiments of the invention generally relate to a fuel combustion process and system. More specifically, embodiments of the invention relate to a two-stage combustion method and system which uses in-situ oxygen generation to combust a fuel and produce a carbon dioxide (CO2) rich flue gas stream from which CO2 can be captured for later utilization and/or sequestration.
Description of the Related Art
Greenhouse gas concentration in the atmosphere has increased significantly over the past years as a result of increasing CO2 emissions. Several mitigation techniques, including, for example, CO2 capture and sequestration (CCS), are being investigated to reduce CO2 emissions in the atmosphere.
One way of reducing CO2 emissions is to capture the CO2 from exhaust flue gases. Several conventional techniques are being developed to capture CO2 before or after combustion. When the combustion of the fuel occurs with atmospheric air, the presence of nitrogen in the air dilutes the CO2 concentration in the flue gases, penalizing the separation of the CO2 from the flue gases.
Several additional conventional techniques are being evaluated to capture CO2 from industrial exhaust flue gases to overcome the limitations of conventional gas separation processes. These conventional techniques, however, are often cost prohibitive to operate. Flue gases produced from conventional combustion are therefore typically treated to capture the CO2. The low concentration level of CO2 in the flue gases results from the use of air (i.e., containing nitrogen) as the source of oxygen to drive the combustion reaction.
Another such conventional technique is chemical looping combustion (CLC), in which oxygen is transferred to fuel without nitrogen interference, thereby generating a CO2 and water vapor stream after the fuel is oxidized. The water vapor can then be easily removed (i.e., through condensation), leaving a higher purity CO2 stream ready for compression, transportation, and/or processing. For at least these reasons, CLC is extensively investigated as a viable means for reducing CO2 emissions. Chemical looping is based on an oxygen carrier that can be oxidized in the presence of air and reduced in the presence of fuel, thereby transferring the oxygen from the air to the fuel. The oxygen carrier is oxidized in an air reactor where it reacts with oxygen present in the air to form metal oxide or oxygen carrier oxide. The oxidized oxygen carrier is fed subsequently to a fuel reactor in a reducing atmosphere where the oxidized oxygen carrier transfers its oxygen to the fuel, thereby allowing the reduced oxygen carrier to be used for another phase of oxidation with air. The fuel is oxidized in the fuel reactor to form combustion products among CO, CO2, and H2O, based on the level of fuel oxidation and whether the chemical looping process is for combustion or reforming.
Several oxygen carriers have been investigated for CLC. It has been found that some oxygen carriers have the characteristic of releasing gaseous oxygen in a fuel reactor, thereby enhancing the oxidation or combustion of fuels. Several oxygen carriers are being investigated for chemical looping processes and no oxygen carrier has been found to address all the challenges faced by conventional oxygen carriers (e.g., oxygen transport capacity, high reactivity, resistance to attrition, cost, lifetime, etc.). It is difficult to achieve full conversion of the fuel in many cases and oxygen polishing have been considered for completeness of the combustion. In such cases, the required oxygen is provided by an external source.”
In-situ oxygen generation, for example, allows the operation of a second oxidation step in which all the fuel can be completely oxidized, maximizing fuel conversion efficiency. Furthermore, by increasing the flue gas temperature generated after oxygen combustion or oxidation, higher live steam temperatures, in the case of power generation, can be generated, thereby increasing the overall thermal efficiency of the process.